358 
WEIGHING  OF  MOIST  PRECIPITATES. 
were  two  pounds  of  water  present,  the  weight  would  be  115-4 
(+8),  and  so  on.  This  enabled  me  to  calculate  a  table,  giving 
in  one  column  the  actual  weight  of  the  pail  when  filled  with 
moist  sulphate,  and  opposite  in  a  second  column,  the  amount  of 
dry  sulphate  corresponding  to  the  gross  weight.  The  weight  of 
dry  sulphate  was  thus  found  as  accurately  as  could  be  desired, 
although  the  amounts  varied  in  practice  from  30  to  105  pounds. 
It  is  nothing  but  an  application  of  the  Archimedean  theorem, 
that,  when  a  solid  body  is  immersed  in  a  liquid  it  loses  a  por- 
tion of  its  weight,  equal  to  the  weight  of  the  fluid  which  it  dis- 
places, or  to  the  weight  of  its  own  bulk  of  the  liquid. 
This,  as  I  suppose,  is  precisely  the  principle  applied  by  Mr. 
Mene.  The  precipitate  he  obtains  by  a  certain  chemical  manipu- 
lation is  a  substance  of  known  composition  and  specific  gravity. 
Supposing  it  to  be  sulphate  of  lead,  and  the  bottle,  when  filled 
with  water  at  the  normal  temperature,  to  weigh  70  grammes  =  50 
grammes  of  water,  and  20  for  tare.  After  introducing  the  pre- 
cipitate and  filling  again  with  water  it  weighed  71-06  grammes. 
Now,  as  the  specific  gravity  of  sulphate  of  lead  is  6-3,  or  as  the 
weight  of  a  cubic  measure  of  sulphate  of  lead  is  6-3  times  that 
of  a  cubic  measure  of  water,  and  as  the  space  of  one  part  by 
weight  of  water  is  taken  up  by  6-2  parts  by  weight  of  sulphate 
of  lead,  it  follows  that  the  quantity  of  sulphate  of  lead  in  the 
bottle,  which  has  taken  up  the  space  of  one  part  by  weight  of 
water,  increases  the  original  weight  of  the  bottle  (filled  with 
pure  water)  by  5-3.  To  find  the  amount  of  water  displaced,  it 
is  only  necessary  to  divide  the  overweight  (1-06  grammes)  by 
5-3  =0-2,  which,  added  to  the  overweight  1.06+0-2  gives  1-26 
grammes  as  the  weight  of  the  precipitate. 
Hence  the  rule,  which  is  of  great  convenience  in  volumetric 
analysis,  that  to  find  the  weight  of  a  moist  precipitate,  which  is  a 
compound  of  known  specific  gravity,  weigh  it  in  a  specific  gravity 
bottle  or  some  other  vessel  of  known  weight,  when  filled  with  water, 
or  any  other  liquid,  at  the  normal  temperature,  again  fill  it  with 
the  water  or  other  liquid,  divide  the  excess  of  the  new  weight  by 
the  specific  gravity  of  the  substance,  less  that  of  the  water  or  other 
liquid  (that  of  water  being  =  1),  and  add  the  quotient  to  the  over- 
weight,  which  gives  the  weight  of  the  precipitate. 
The  editor  of  the  Jahresbericht  appears  to  have  overlooked 
